With no particular fact-based argument to offer up to counter this point, swamp, my gut feeling is that your figure is off by a wide margin on the low side.

Also, regarding estimates, I saw your disclaimer and realize you were just throwing out numbers. That being said, if we wanted to get closer to real figures I would think that it would be pretty safe to start with the premise that there will be a higher cost per pound saved as weight tends toward zero. We can even put an asymptote at around 2000 lbs or so. I.e. it would cost an infinite amount of money to build a 2000 lb M3 (read: impossible).

Edit: You know what, I guess I didn't appreciate the fact that you were talking about material costs only. So disregard my statements as a counterpoint. However, they do make for a reasonable secondary supporting argument. Furthermore, I will suggest that material costs are among the least significant considerations. Though to be perfectly honest, I really have no idea what drives costs of carbon fiber. I suppose I should hit the search engines and see if I can find some information on it because you have piqued my interest.

Ball is in your court here. Traditional layered, oriented woven or long fiber materials are very expensive. Both components are expensive, the CF itself as well as the labor. BMW is obviously attacking both fronts but the savings will occur incrementally. Reducing the existing cost of material by half is a huge deal.